Synthesis, Crystal and Electronic Structure of the Quaternary Magnetic EuTAl₄Si₂ (<i>T</i> = Rh and Ir) Compounds

Single crystals of the quaternary europium compounds EuRhAl₄Si₂ and EuIrAl₄Si₂ were synthesized by using the Al–Si binary eutectic as a flux. The structure of the two quaternary compounds has been refined by single crystal X-ray diffraction. Both compounds are stoichiometric and adopt an ordered derivative of the ternary KCu₄S₃ structure type (tetragonal <i>tP</i>8, <i>P</i>4/<i>mmm</i>). The two compounds reported here represent the first example of a quaternary and truly stoichiometric 1:1:4:2 phase crystallizing with this structure type. In light of our present results, the structure of the BaMg₄Si₃ compound given in literature as representing a new prototype is actually isotypic with the KCu₄S₃ structure. Local spin density approximation including the Hubbard <i>U</i> parameter (LSDA + <i>U</i>) calculations show that Eu ions are in the divalent state, with a significant hybridization between the Eu 5d, Rh (Ir) 4d (5d), Si 3p and Al 3p states. Magnetic susceptibility measured along the [<i>001</i>] direction confirms the divalent nature of the Eu ions in EuRhAl₄Si₂ and EuIrAl₄Si₂, which order magnetically near ∼11 and ∼15 K, respectively

Synthesis and Characterization of ReS₂ and ReSe₂ Layered Chalcogenide Single Crystals

We report the synthesis of high-quality single crystals of ReS₂ and ReSe₂ transition metal dichalcogenides using a modified Bridgman method that avoids the use of a halogen transport agent. Comprehensive structural characterization using X-ray diffraction and electron microscopy confirm a distorted triclinic 1<i>T</i>′ structure for both crystals and reveal a lack of Bernal stacking in ReS₂. Photoluminescence (PL) measurements on ReS₂ show a layer-independent bandgap of 1.51 eV, with increased PL intensity from thicker flakes, confirming interlayer coupling to be negligible in this material. For ReSe₂, the bandgap is weakly layer-dependent and decreases from 1.31 eV for thin layers to 1.29 eV in thick flakes. Both chalcogenides show feature-rich Raman spectra whose excitation energy dependence was studied. The lower background doping inherent to our crystal growth process results in high field-effect mobility values of 79 and 0.8 cm²/(V s) for ReS₂ and ReSe₂, respectively, as extracted from FET structures fabricated from exfoliated flakes. Our work shows ReX₂ chalcogenides to be promising 2D materials candidates, especially for optoelectronic devices, without the requirement of having monolayer thin flakes to achieve a direct bandgap